1. Field of the Invention
The present invention generally relates to a transmission line method and apparatus, and more particularly to a transmission line monitoring method and apparatus which can pinpoint where a fault occurs among apparatus connected by optical transmission lines.
2. Description of the Related Art
Recently, a network of optical fibers has been laid near to user's houses. This kind of optical fiber network has a maintenance management function of roughly specifying where a fault is when the fault occurs.
For example, the maintenance management function is described with reference to FIG. 1 showing a conventional transmission line monitoring apparatus. The conventional apparatus comprises a service station 10, a user house 20, an optical fiber transmission down line 40 and an optical fiber transmission up line 42.
The service station 10 includes a station apparatus 12 such as an optical terminal apparatus disposed therein. The user house 20 includes a terminal apparatus 22 and an in-house apparatus 24 such as a DSU (Digital Service Unit) with an optical/electric converting function. In general, the station apparatus 12 is referred to as a host apparatus and the in-house apparatus 24 as a lower apparatus.
For example, the station apparatus 12 and in-house apparatus 24 both have return points for distinguishing a location of a fault. In particular, the in-house apparatus 24 has a return point near the terminal apparatus 22, and, at this return point, it can be distinguished whether the fault has occurred in the terminal apparatus 22 or on the host side including the in-house apparatus 24 and the host apparatus 12.
At the return point of the in-house apparatus 24, an optical-to-electric converter 28 converts an optical signal received from the station apparatus 12 into an electric signal. The converted electric signal is transmitted to an electric/optical converter 30 via a returning processor 26. Then the electric/optical converter 30 converts the received electric signal into an optical signal and the converted optical signal is further transmitted to the station apparatus 12.
In a case in which an out-of-phase of a received signal or an abnormality of the transmission up line 42 is detected in the station apparatus 12, it can be confirmed that a fault has occurred in the in-house apparatus 24 or in the optical fiber transmission up line 42. In this case, it is needed to go to the user house 20 to change the in-house apparatus 24 disposed therein. After that, if the fault is removed, then it can be determined that the fault occurred in the in-house apparatus 24. If the fault is not removed even after the in-house apparatus 24 is changed, then it is needed to go to check the optical fiber transmission up line 42.
In a case in which an out-of-phase of a received signal or an abnormality of the transmission down line 40 is detected in the user house 20, it can be confirmed that a fault has occurred in the in-house apparatus 24 or in the optical fiber transmission down line 40. In this case, it is needed to go to the user house 20 to change the in-house apparatus 24 disposed therein. After that, if the fault is removed, it can be determined that the fault occurred in the in-house apparatus 24. If the fault is not removed even after the in-house apparatus 24 is changed, then it is needed to go to check the optical fiber transmission down line 40.
As mentioned above, at the return point of the in-house apparatus 24, it can be distinguished whether a fault position is in the terminal 22 or on its host side including the in-house apparatus 24. Further, at the return point of the station apparatus 12, it can be distinguished whether a fault-occurring position is on the host side of the station apparatus 12, or on the lower side including the in-house apparatus 24, the optical fiber transmission down line 40 and the optical fiber transmission up line 42.
However, at the return points of the conventional transmission line monitoring apparatus, a fault position cannot be distinguished between the in-house apparatus 24 and the optical fiber transmission down and up lines 40, 42.
Hence, in a case where a fault occurs in any one of the in-house apparatus 24, the optical fiber transmission down line 40, and the optical fiber transmission up line 42, a position of the fault cannot be accurately determined without performing a field survey. Performing such a field survey increases man-hours.
Furthermore, if a power supply of the in-house apparatus 24 in the user house 20 is disconnected, the conventional transmission line monitoring apparatus cannot work normally.